NA I as a tracer of H I in the diffuse interstellar medium

From a compilation of interstellar atomic sodium and neutral hydrogen absorption measurements, together with new high signal-to-noise ratio Na I observations and detailed analysis through a profile fitting method, the column-density correlation N(Na I) versus N(H) has been derived. Extending now over more than three orders of magnitude, especially toward low-column density clouds, a slope of unity is found instead of the previous quadratic dependence. Still valid for mean space densities, this new result leads to a deduction that, in the diffuse interstellar medium, the product of the recombination coefficient of Na II and the electron space density, alpha n(e), is nearly constant. The n(e) evaluations are consistent with the three-phase model of the interstellar medium, while at high densities, the shielding of UV photons by dust becomes the dominant factor.